Note: Descriptions are shown in the official language in which they were submitted.
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DAMPING SOLUTION FOR OFFSET PRINTING PLATES AND METHOD
AND DEVICE FOR THE PREPARATION THEREOF
Technical sector of the invention.-
The invention relates to a humidifying solution
for offset print sheet and to a procedure and to a
device for the preparation of said humidifying solu-
tion.
Specifically, the humidifying solution of the
invention is of the type that are formed by between
95% and 97% of water and between 3% and 50 of an
additive which contains sodium citrate, acetic acid,
citric acid and sodium benzoate.
The procedure to obtain the humidifying solution
to which the invention refers is of the type which
contains a preliminary phase of oxygen adsorption by
the means of projecting a thrust, over the free
surface of the humidifying Jolution, of recycled
fractions of the humidifying solution.
The device for the preparation of the humidify-
ing solution according to the invention is of the
type which contains a main container reservoir for
the solution, an outlet pipe for the solution pro-
vided with a first pump that recycles part of the
solution back to some means for thrust propulsion of
the recycled fraction over the free surface of the
solution, which is present in an annex container
reservoir, main and independent from it, a feeding
pipe which communicates said annex compartment with
the humidifying bucket for the humidifying cylinder
of an offset printing machine and a feedback pipe for
the solution in the humidifying bucket to main con-
tainer reservoir.
State of Lhe art.-
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The offset printing is an indirect printing
fashion in which three cylinders are used: the print-
ing sheet is mounted on the first cylinder; the
second cylinder transmits the image to a second
cylinder, which is covered by a rubber layer, which
prints the paper which traverses between itself and a
third cylinder which is only for pressing purposes.
The pressing sheet is covered with photo-sensible
emulsions in the shape of colloids which, when ex-
posed to light, polymerize creating some surface
zones which are lipophilic or similar to printing
ink, which are fat bodies and hidrophilic metallic
zones without images, which repel the ink.
A first set or rollers provide the printing ink
to the first cylinder that the sheet has and the
other set of rollers wets the sheet. The image zones
accept ink, while the blank ones reject it. The
addition of wetness to the sheet is achieved using
what are called wetting systems.
The offset printing is a system based in surface
physic and chemical phenomena, that is it is not a
system based on relief, like typography, flexography,
tampography, etc., but it is a planographic system in
which the phase previous to the transferring of the
ink to the paper consists in chemical attractions and
repulsions between the different lipophilic and
hidrophilic substances. Therefore, both the water
'and the ink must have a set of adequate physical and
chemical characteristics that allow that the process
be the right one.
Around this idea, and keeping in mind that the
water with which the offset machines work is regular
network water, there is the need to add some special
additives to the water so that it gets determined
properties in pH and surface tension, among others.
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Among the wetting systems, the water alcohol
system can be pointed out. It consists in adding,
besides some additives, between 10o and 12% of iso-
propil alcohol to the water. The water alcohol system
is the most developed and extended one because the
quality parameters with this system are far better
than with any other system, for example those called
in the "integral" and "conventional" techniques. The
basis for this good behavior is that the isopropyl
alcohol provides a good interfacial tension between
the water and the ink which brings the consequence a
better water in the ink emulsion, so the printed ink
is more vivid than with any other known wetting
system.
Some other additional advantages are that the
wetting system with isopropylic alcohol provides a
higher viscosity that allows a more uniform image
transference, a faster drying speed and a decrease in
the surface tension that allows the water layer over
the sheet to be very thin and to have a lower ten-
dency to the mechanical emulsion.
However, for some years now, very rigorous
studies are being conducted on the possible toxicity
that the isopropylic alcohol based systems can pres-
ent due to a prolonged exposure. In this sense, some
countries have absolutely forbidden its use, while in
some others its use has been largely restricted and
in our country it is on its way to being forbidden
for good.
Apart from toxicity, they present storing diffi-
culty problems and danger of inflammability.
Many manufacturers have unsuccessfully tried to
formulate additives that satisfy the aforementioned
quality needs and refrains completely from using the
isopropyl alcohol. The reason of the failed tries is
in essence that the need to have to reduce the sur-
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face tension to avoid that the water ink emulsion to
levels which can be compared to the water alcohol
system obligates the use of tensoactives of group
III. A larger concentration of these tensoactives
originates a decrease in the interphase by affinity
between the ink fat acids and the tensoactives hydro-
carburated strings. The tensoactives cause a great
deal of foam and, because they are no volatile, they
remain in the ink rollers, thus accumulating and
causing operating problems.
The former, together with the impossibility to
form stable colloids based on other substances that
reduce the surface tension and maintain a larger
water ink interphase are the main reasons why it has
been impossible to substitute the isopropyl alcohol.
European patent number EP-A-0 770 501 introduces
some improvements in wetting systems for offset
printing, consisting in feeding the wetting system
with a dissolution based on water and some determined
additive. The dissolution is subjected to some
collision phenomena during which it acquires, due to
an adsorption process, an important amount of oxygen,
with which it is achieved that the dissolution that
wets the printing sheet, based on which the impres-
sion is made, acquires a large amount of free oxygen,
thus increasing the water ink inter-phase.
Also, the EP-A-0 770 501 describes the device
for the preparation of the solution, which consists
of a main container reservoir for the solution, an
outlet pipe provided with a first pump that recycles
part of the solution towards some means for thrust
projection of the recycled fraction over the free
surface of the present solution in a compartment
annex to the main container and independent from it,
a feeding pipe, which communicates said annex com-
partment with the wetting bucket of the wetting
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cylinder of an offset printing machine and a feedback
pipe for the wetting bucket solution to the main
container reservoir.
Said improvements and device, even if they have
a correct functionality and provide a good final
quality, lacks the collision of the recycled fraction
against the free surface and does not achieve a
constant adsorption isothermal on the surface of the
solution, which would be desirable for getting a
larger uniformity of extension of the humidifying
layers over the printing sheet.
The goal of the invention is to provide an
effective solution to all problems listed above.
Summary of the Invention.-
To that extent, the invention makes public the
humidifying solution, which in essence is character-
ized in that the aforementioned additive contains
between 0.0% and 0.5% in weight of sodium hexameta-
phosphate, between 0.0% and 1.5% in weight of ascor-
bic acid, between 0.0% and 1.0% in weight of sodium
benzoate, between 0.0% and 1.0% in weight of citric
acid, between 2% and 10% in weight of glycerin,
between 0.0% and 0.1 % in weight of limonal, between
50% and 75% in weight of sorbitol, between 1% and 2%
in weight of acetic acid, between 3% and 7% in weight
of sodium citrate, between 0.0 and 1.0% in weight of
sodium oleate and between 15% and 30% in weight of
osmotic water.
According to one other feature of the humidify-
ing solution of the invention, the aforementioned
additive is constituted by 0.1% in weight of sodium
hexametaphosphate, 0.3% in weight of ascorbic acid,
0.211 in weight of preventol, 0.2% in weight of sodium
benzoate, 0.2% in weight of citric acid, 5% in weight
of glycerin, 0.05% in weight of limonal, 65% in
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weight of sorbitol, 1.5% in weight of acetic acid, 50
in weight of sodium citrate, 0.2% in weight of sodium
oleate, and 22.251 in weight of osmotic water.
The invention also has the goal to a method to
obtain the aforementioned humidifying solution,
characterized in that the solution is forced to flow
at high speed by a pipe with a helicoidal configura-
tion and is projected thus forming a 90-degree solid
angle output cone, in such a way that a part of the
particles of the projected thrust collide against the
free surface of the humidifying solution, obtaining
on said free surface a isothermal of constant adsorp-
tion.
According to one other feature of the invention,
the humidifying solution is projected at a lineal
speed between 1.3 m/s and 220 m/s and at a pressure
up to 1,500 kg/cm2.
According to one other feature of the invention,
said lineal speed is 60 m/s and said pressure is 250
kg/cm2.
Another goal of the invention is a device for
the preparation of a humidifying solution according
to the invention using the method described above,
which in essence is characterized in that the afore-
mentioned projection means are constituted by an
injector which makes the solution turn at high angu-
lar speeds in a helicoidal trajectory and subject to
very high pressures, so that the solution is pro-
jected forming a output cone of 90 degrees of solid
angle, so that a part of the particles of the pro-
jected thrust collide against a deflecting surface
and the rest of the particles collide against a free
surface of the humidifying solution an isothermal of
constant adsorption.
Preferably, the aforementioned injector has an
initial acceleration part in a helicoidal trajectory
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and right after it a second diffusion part of toroi-
dal configuration, so that the humidifying solution
is projected over said free surface and against said
deflecting surface at a lineal speed between 1.3 m/s
and 220 m/s and at a pressure up to 1,500 kg/cm2.
In an actual realization of the invention, the
lineal speed is 60 m/s and said pressure is 250
kg/cm2.
According to one other feature of the invention,
said deflecting surface makes an angle of between 0
degrees and 30 degrees with said liquid free surface.
Preferably, said deflecting surface (4) makes a
15-degree angle with the said liquid free surface.
Brief description of the drawings.-
Next, there is a description of an actual reali-
zation of the invention, although not exclusive, for
whose better understanding a set of drawings is
accompanied, give merely as non-limiting examples, in
which:
Figure 1 is a schematic view of a device according to
the invention; and
Figure 2 is a section representation of an injector
according to the invention in which a cut has been
made for a better understanding.
Detailed description of the drawings.-
In said drawings, it can be seen that the device
according to the invention comprises a main container
reservoir 1 for the humidifying solution 18 for the
wetting cylinder 14 of an offset printing machine.
The set has an compartment 5 annex to the main reser-
voir 1, destined for containing a fraction 19 of the
recircled humidifying solution 18. The free surface
13 of the annex compartment 5 is at a higher level
than the free surface 20 of the main reservoir 1.
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The main container reservoir 1 is connected to
an output pipe 2 for the humidifying solution 18,
provided with a first pump 7 that pushes to the annex
compartment 5 the recircled fraction 19, which is
projected at high speed and pressure by the means of
an injector 3, in such a way that the interior of the
compartment 5 is subjected to pressure.
The annex compartment 5 is provided in its
interior with a deflecting surface 4, destined for
receiving the impact of part of the recycled frac-
tion thrust 19 of the humidifying solution 18 when it
is projected by the injector 3.
It has been proven that an effective way to take
into practicality the device of the invention is the
one in which the deflecting surface 4 makes an angle
of between 0 degrees and 30 degrees with said liquid
free surface, with the optimal solution being that
whose angle is 15 degrees.
In Figure 2, it can be seen that the injector 3
has a first acceleration part 16 in helicoidal tra-
jectory and, right after it a second diffusion part
17 with toroidal configuration, which makes the
solution turn at high angular speeds in a helicoidal
trajectory and at high pressures. The humidifying
solution 18 is projected by injector 3 forming a
output cone 15 of 90 degree of solid angle. Part of
the particles of the thrust collide against the
deflecting surface 4 and the rest of the particles,
against the free surface 13 of the humidifying solu-
tion, obtaining on said surface a constant adsorption
isothermal. Injector 3 can provide a lineal speed
between 1.3 m/s and 220 m/s and a pressure of up to
1,500 kg/cm2, preferably 60 m/s and 250 kg/cm2,
respectively.
The annex compartment 5 is communicated, via a
feeding pipe 8, with the one with which it connects
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with its lower part, with a wetting bucket 10 of the
wetting cylinder 14 of the offset printing machine,
and to a feedback pipe 12 for the humidifying solu-
tion 18 from the wetting bucket 10 to the main con-
tainer reservoir 1. The fraction of the humidifying
solution that goes from the annex compartment 5 to
the wetting bucket 10 is pushed by a second pump 9.
The feedback pipe 12 is provided with a third pump
11, for the remaining fraction of the humidifying
solution 18 that has not been incorporated to the
printing sheet.
The annex compartment 5 is also communicated
with the main reservoir 1 with the aid of a spiller 6
that spills because of gravity and in collaboration
with the present pressure within the annex compart-
ment 5.
The described device is particularly designed
for the execution of the method of the invention for
the preparation of the humidifying solution 18 for
the offset printing sheets.
Said humidifying solution 18 is formed by be-
tween 9501 and 97% of water and between 3% and 50 of
an additive which contains between 0.0i and 0.5o in
weight of sodium hexametaphosphate, between 0.01 and
1.506 in weight of ascorbic acid, between 0.0% and
1.01i in weight of sodium benzoate, between 0.0% and
1.0% in weight of citric acid, between 2% and 10% in
weight of glycerin, between 0.0% and 0.1 o in weight
of limonal, between 50o and 75% in weight of sorbi-
tol, between lo and 2% in weight of acetic acid,
between 3% and 7% in weight of sodium citrate, between
0.0 and 1.0 o in weight of sodium oleate and between
15% and 30% in weight of osmotic water.
The composition of the cited additive is consti-
tuted by 0.1% in weight of sodium hexametaphosphate,
0.3% in weight of ascorbic acid, 0.2% in weight of
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preventol, 0.2% in weight of sodium benzoate, 0.2o in
weight of citric acid, 511 in weight of glycerin,
0.05% in weight of limonal, 65% in weight of sorbi-
tol, 1.5% in weight of acetic acid, 5 s in weight of
5 sodium citrate, 0.2o in weight of sodium oleate, and
22.25% in weight of osmotic water.
The method of the invention is based on modify-
ing the molecular structure of the water for the
humidifying by forming gas surfaces of 02 similar to
10 those formed with colloids in association, achieving
that all the humidifying solution 18 layer applied to
the printing sheet acquires a large amount of free
oxygen.
In essence, on the surface 13 of the humidifying
solution 18 some "vapor gas surfaces" are formed,
which are obtained by subjecting the solution to
colliding phenomena at high speed and pressure;
projecting with the means of injector 3 a recycled
fraction of the solution, this volume being the one
that feeds in a continuous way the wetting system.
Next, an explanation for the physical and chemical
phenomena which rule the aforementioned structural
modification of water is given.
A colloid differs from a dissolution basically
in the size of the particles: from 0,1 mm to 1 mm in
the first case and between 1 mm and several mm in the
case of the colloids. There exist three types of
colloids, namely: colloidal dispersions, macromole-
cule solutions and association colloids.
Next, the fundamentals of the association col-
loids are explained briefly, which are, among the
three types listed, the ones that pertain to the
physics and chemistry of the offset printing.
The substances that in solution get the name of
association colloids are basically soaps and humidi-
fying agents.
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In the conventional additives that are currently
used in the printing industry, humidifying agents in
one or several varieties are found. In a generic
way, it can be said that these additives generally
are aliphatic or mixed (aliphatic and aromatic)
strings along with very strong polar groups to make
them soluble.
Taking into consideration their insolubility, in
absence of said additives, of these strings in disso-
lution, the situation thermodynamically mote stable
is the grouping in what is called as micelles. And
on the surface, the situation of a lesser energy is
verified with the polar group in dissolution and the
hydrophobic string outside the water.
These surfaces that have the property to present
less surface tension than pure water are known as
"gas vapor surfaces". The interactions that exist are
always solute-solvent, because if only solute-solute
interactions existed, it would be before homogeneous
surfaces, or even condensed, which are not the right
kind for the case that occupies us.
Taking the former into consideration, it is un-
derstood that the creation of any new surface in an
association colloid originates instantaneously a gas
vapor surface, the general base for the working of
the wetting additives.
If ink is spread on a surface of pure water, the
ink will rapidly spread on the water forming a layer.
This happens because the so-called "Initial Exten-
sion Coefficient (S)"is zero or positive. The ini-
tial extension coefficient is given by the formula:
S = yw - (yw /t - yt )
where yw is the tension component in the inter-
phase water-ink with respect to the horizontal, t is
the size of the bubbles created and yt the tangent
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component force in the interphase water-ink, or to
the contact angle.
When a conventional additive is added to water,
it happens that there is no extension, in part be-
cause the surface tension has been decreased and in
part because yw has ceased to be null and has taken
the value of 30 degrees. Therefore, these two causes
make the system to be stable in the condensed form.
However, in this new case, the value of yw/t has
decreased. That is, the addition of a humidifying
has reduced the surface tension.
In theory, it could be assumed that adding only
alcohol to water and thus reducing the surface ten-
sion something similar should happen. However, it
does not work that way. In any event, it is neces-
sary the presence of a humidifying for the system to
be stable in condensed state. The tension in the
interphase with only alcohol is not large enough to
produce this effect.
For that purpose, like it has already been said,
the presence of an additive that in a similar way to
prior case modifies the contact angle is required.
However, the value of yw/t with alcohol is compara-
tively rather bigger and this is the fundamental
cause for its good working.
Because it is impossible to form stable colloids
(without tensoactives) to reduce the surface tension,
and knowing that all the current existing substitu-
tions have a behavior which is not particularly good
or acceptable, the principles of the invention
achieve solving the problem by the means a completely
different approach.
It has been proceeded to try a similar adsorp-
tion to the Langrnuir Type 1, that is, an adsorption
in which the adsorption speed of a gas over a solid,
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v, depends on a constant Kl, of the fraction of
covered surface, D, and on the pressure, P:
v = K1 (1 - D) * P
In the case of the system of the invention, be-
sides the pressure, because it is an adsorption over
the surface of a liquid, the speed of condensation
has been proven to depend on a factor related to the
number of collisions of air bubbles and also on their
size of over the liquid surface. Therefore, the speed
can be represented by the following formula:
v = K3 (1 - D) * P* C
Where
c = f(N, t)
Where N is the number of collisions per time
unit.
Where N is the number of collisions per time unit.
The system of the invention does not work over a
Gibbs isothermal, in which the adsorption processes
are stable and always measurable depending on the
concentration of the humidifying and on the variation
of the surface tension with its concentration, as it
would be according to the following equation:
R = C/RT (dt/dc) T
But it produces a reversible adsorption isother-
mal in which:
V = K2 * D
And in which an internal fraction of volume of
the humidifying solution does not present these
conditions at any moment, but only the free surface
13 of the humidifying solution present in the annex
compartment 5 originates this phenomenon.
Therefore, it has been determined that the in-
crease of the interfacial tensions derived from
function C can be obtained from experiments in which
it has been worked with the following parameters:
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a) different formulations, with different sensibili-
ties of the emulsion to the air;
b) different speed of the fluid under pressure, over
the volume to deal with;
c) different ratio surface-volume of the fluid to
deal with;
d) different collecting zones of the fluid to pump to
the water tray 10 (Figure 1); and
e) different additional treatments of turbulence over
the water tray 10, only in specific cases.
The procedure according to the invention
achieves that the water that the wetting rollers 14
collect from the water trays 10 gets to 02 adsorption
conditions to the sheet, adequate for, on one side to
create interphase tension higher than that of the
water alcohol system, and on the other a layer of
water so thin it does not cause excessive emulsion.
This phenomenon happens thanks to the 02 adsorp-
tion on the water surface in contact with the wetting
roller 14, gets transmitted with the rest of the
system's wetting roller, and from those to the hydro-
philic zone of the sheet.
An expert on this sector of the technique is
aware of the fact that the additive does not incorpo-
rate any tensoactive from group III. A wetting
dissolution with a tensoactive and without adsorption
treatment produces spreading of the ink over the
Vuater as a consequence of that the surface tension of
the water does not decrease, and therefore the exten-
sion coefficient is zero or positive, which is not
desirable.
However, the method of the invention, once the
02 adsorption over the surface of the wetting water
has happened, not only there is no extension but the
speed of flocculation of a thin layer of ink (tested
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in a laboratory and machine) is much larger than in
water-additive-alcohol.
This means that the extension coefficient is
less in the system according to the invention than in
the conventional water-alcohol, which means a clear
advantage.
The printers know from experience that in the
inmerser roller 14 of a water-alcohol wetting system,
the chromate must present, as it runs, an aspect of a
thin water layer, which is indicative that, the sur-
face tension is the ideal mixture. When the surface
tension increases (for instance, because the alcohol
concentration has decreased) this thin layer breaks
causing water extension problems over the rollers and
the sheet, obligating them to increase the system
water flow and the speed.
The adsorption on the surface that the system of
the invention produces, creates a situation similar
to that of the alcohol, and even when the resulting
surface tension of the additive is close to 72 di-
nas/cm it is deduced that when the system of the
invention has produced enough surface 02 adsorption,
the same phenomenon happens over the inmerser roller
as a consequence of the fact that the surface tension
over said roller has decreased.
From the experimental development, test under-
taken in workshops and in real tests the following
tesults are obtained, which will be clear for an
expert on the technique:
- isopropylic alcohol total elimination;
- more uniformity of the extension of the ink over
the sheet;
- a thinner and more homogeneous over the sheet;
- lower emulsion of water over ink;
- better extension of ink over the set with less
water and less paper particles;
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- colors more vivid;
- ink savings of up to 14% in normal offset and even
higher in UV;
- higher drying speed of the ink;
- reduction and in some cases of anti-spot dusts;
- better "wet over wet" impression in a multicolor
machine with a higher ink transparency;
- important improvement of the "additionability" and
"trapping";
- better "impasted";.
- better contrast;
- better gain point;
- less stops for cleaning and the rubber layer; and
- better reproduction of the weft between 1% and
990, with very good results with special wefts,
such as the "stochastics".
Although the present invention has been explained
hereinabove by way of a preferred embodiment thereof,
it should be pointed out that any modifications to
this preferred embodiment within the scope of the
appended claims is not deemed to alter or change the
nature and scope of the present invention.
